1. Field of the Invention
The invention relates to an encoder for measuring devices to digitally measure a length or an angle with high accuracy and high resolution.
2. Description of the Prior Art
Generally, a linear encoder or a rotary encoder has been known as an encoder in use for a measuring device for digitally measuring a length or an angle. The linear encoder is used for the measurement of a length while the rotary encoder for the measurement of an angle. Those encoders are essentially the same, except that the former has a linear code pattern and the latter a circular code pattern. Those encoders are categorized into an optical encoder and an electromagnetic encoder using magnetic or electrostatic induction.
In manufacturing those encoders, there is a limit in the graduations of the scale plate and therefore the improvement of the measuring accuracy is restricted. This restriction is inevitable irrespective of whether the scale is read out optically or magnetically. The minimum measuring limit, i.e. the minimum scale value, entirely depends on the limit attendent on the fractionalization of the scale. To obtain a more precise measurement or improve a resolution of the measurement (to measure smaller scale values than the minimum scale value), it is necessary to interpolate the scale division. To this end, various measuring methods using the interpolation have been proposed.
In a conventional encoder adapted for the measuring device using the interpolation method, a couple of sensors are provided to read out the information of optical or magnetic gratings corresponding to a scale on an encoding plate a code plate. Those sensors read out a couple of information differing in phase by 90.degree., i.e. 1/4 pitch of the scale on the code plate. By processing those two pieces of information, the interpolation of the scale division of the grating pattern is made. The following three interpolations are known. The first interpolation consists in that a couple of sine wave signals with different phases are wave-shaped into rectangular wave signals with references of the cross points where the two sine wave signals cross zero potential. By counting the leading and trailing edges of the two pulse trains of rectangular pulses, one period (one pitch) is interpolated. The second interpolation consists in that a couple of output signals with 90.degree.--different phases produced from an encoder are used. In this interpolation, the amplitude ratio therebetween is employed for the interpolation of one period. The third interpolation consists in that two signals with 90.degree.--different phases are modulated by a carrier with a given frequency. The interpolation of one period is made by detecting a phase difference between the non-modulated composed signal and the carrier signal. The improvement of the resolution obtained by those interpolations is at most 1/4 pitch.
The interpolations described above, however, require an analog signal processing circuit. An extremely complicated and costly circuit therefore is needed for analog processing, with insufficient accuracy. On the contrary, if the omission of such processing circuit is required, the pattern pitch of the coading boad should be formed minutely. In this case, however, the production of the coading boad is difficult and is very expensive.